Proper firefox installation should create symlinks in $HOME/.mozilla/plugins directory.
You should find there libflashplayer.so or valid symlink to libflashplayer.so
1. Make sure you have www-plugins/adobe-flash installed
2.
cd $HOME/.mozilla/plugins
ln -s /usr/lib/mozilla-firefox/plugins/libflashplayer.so
And that’s it.. in my case I didn’t even had to restart browser.
One may ask why creating symlink to other symlink.
This allows firefox instalation assume that /usr/lib/mozilla-firefox/plugins/libflashplayer.so is always valid URL for flash plugin while system administrator can change this symlink to all users using
eselect java-nsplugin